In mobile units having articulated booms with a movable lift platform at the end of the upper boom, there has been some considerable difficulty in fitting the unit with a device which will cushion movement of the lower boom into its final rested position. Moreover, in the hydraulic devices presently being used, because there is a positive retracting force exerted on the lower boom the retracting device can at times be destructive as it tends to draw positively upon the lower boom, pulling it with destructive force against the final resting place for the boom in preparation for transport.
Hydraulic devices have been proposed for incorporation into the hydraulic systems of units to effect more controlled positive retracting force on the lower boom. These devices are for the most part used in conjunction with the pilot operated check valve and cylinder, and therefore interrupt the normal operation of the hydraulic device. Such devices must be "built into" the hydraulic system at the time of its initial manufacture in order to produce an integrated system.
What the present invention proposes is a much simpler and more reliable system for controlling the actuation of the fluid motor (power cylinder) associated with the lower boom, the unit being inserted between the main control valve and the fluid motor. There is no interference with the pilot operated check valve and consequently the inserted unit is made up of hydraulic "package" which can be retrofitted into existing hydraulic systems having motors or power cylinders for retracting the lower boom. The present system will operate to accomplish its intended purpose but in no way will interfere with normal operation of the power cylinder for the lower boom. The device or unit imposes upon the operation of the hydraulic cylinder the special condition that as the lower boom approaches horizontal positions, it will, at a critical angle, cause normal hydraulic application to cease, and thereafter retracting force will be diminished and a much reduced pressure will be effective for operating the lower boom from the critical angle thereafter to the full horizontal stored position for transit.